Lasers are widely used as sources of monochrome and coherent light in telecommunication industry and other industries. Known laser types include for example gas discharge lasers, laser diodes and fibre lasers.
Fibre lasers provide an alternative to conventional bulk lasers and offer high efficiency, high beam quality, excellent heat dissipation and are typically of a relatively small size.
A typical fibre laser may comprise an erbium doped amplifying medium that is optically pumped by a laser diode. The optical pumping results in inversion and suitable light that is guided through the amplifying medium causes stimulated light emission. The amplifying medium may be positioned between two Bragg gratings so that a laser cavity is formed and monochrome laser light is generated.
For some applications pulsed laser sources are required and recently a mode-locked figure-eight laser has been developed. The figure-eight laser includes a non-linear optical fibre and an amplifying medium in a first loop, which provides an amplifying loop mirror for light that is guided in an optically coupled second loop.
A phase shift that light will experience when guided through the non-linear optical fibre of the first loop depends on the light intensity and consequently light that passes through the amplifying medium before passing through the non-linear optical fibre will experience a phase shift that is different to that of light guided in an opposite direction. Interference of light guided in both directions and a suitable optical coupler is used to generate pulsed light having desired properties.
In a known passively mode-locked laser a pulse that initiates the lasing originates from optical noise fluctuation. In an actively mode-locked laser, mode-locking typically is generated using an optical modulator which is electronically controlled and is used to control a pulse.
In particular passively mode-locked figure-eight fibre lasers have been of interest for many applications as they are of relatively low cost and simple construction. However, as the initiating pulse originates from noise, the lasing performance, which depends on the properties of the initiating pulse, is of poor repeatability and the laser often has unpredictable performance.
The present invention provides technological advancement.